The present invention relates to an electrostatic plotter or similar color image forming apparatus for forming an image on a paper web or similar recording medium being transported under tension by a transport roller pair by a head while sensing a register mark recorded on the medium.
Color image forming apparatuses of the type recording an image on a recording medium, i.e., paper web being transported under tension by multiple roller pairs are extensively used today and include an electrostatic color plotter. It is a common practice with this type of apparatus to record a register mark in a predetermined position on a recording medium and sense it by, for example, a photosensor. The transport of the medium is controlled on the basis of the register mark to form color components expected to form a color image in accurate register on the medium. To enhance the sensing accuracy of the photosensor, the register mark is condensed by a lens and then sensed by the photosensor. However, the problem is that the medium in transport is apt to move up and down at the position where the photosensor is located, changing the distance between it, i.e., the register mark provided thereon and the lens. Then, the photosensor fails to sense the register mark accurately. A head for recording an image on the medium has to be spaced apart from the medium by a predetermined distance. Should this distance be inaccurate, especially in the main scanning direction, the quality of reproduced images would be critically degraded. In practice, however, the distance between the recording medium and the head changes when the medium is set on the apparatus or in the event of maintenance of the head. Specifically, in such an occasion, since a guide member for guiding the medium is rotated about, for example, a hinge to a retracted position, the guide member cannot be accurately restored to the original or operative position due to the play of the hinge.